1. Field of the Invention
The present invention relates to variable capacitance devices that change a capacitance using a microelectromechanical system (MEMS) component driven by an electrostatic force.
2. Description of the Related Art
Recently, some variable capacitance devices use a MEMS component that is driven by an electrostatic force (refer to Japanese Unexamined Patent Application Publication No. 2006-210843 and Japanese Unexamined Patent Application Publication No. 2008-182134).
FIGS. 1A to 1C illustrate an exemplary configuration of an existing variable capacitance device.
A variable capacitance device 101 includes movable plates 102 and 103. The movable plates 102 and 103, which have a double supported beam structure made of a conductive material, are arranged so as to be opposite each other. The movable plate 103 has a convex surface that is provided with a dielectric layer 104 and faces the movable plate 102. When a DC voltage is applied between the movable plates 102 and 103, an electrostatic force is generated which attracts the movable plates 102 and 103 toward each other. Thus, the distance between the movable plates 102 and 103 is decreased and the leading portion of the convex surface of the movable plate 103 comes into contact with the movable plate 102 with the dielectric layer 104 therebetween, whereby the capacitance of the variable capacitance device 101 is increased. When the DC voltage is changed, the contact area between the movable plates 102 and 103 is changed, whereby the capacitance is changed in accordance with the contact area.
When the existing variable capacitance device is provided at a point along a signal line through which a high-voltage RF signal passes, an electrostatic force is generated at an electrode to which an RF signal is applied in the variable capacitance device. This electrostatic force prevents proper deformation of the beam structure due to a DC voltage and, thus, the variable capacitance becomes unstable. When the spring constant of the beam structure is relatively high, deformation of the beam structure due to the RF signal can be suppressed. However, the deformation of the beam structure due to the DC voltage is suppressed at the same time and, thus, the DC voltage must be increased. As a result, for example, a voltage step-up circuit is required and the circuit structure of the variable capacitance device becomes more complex.